1. Field of the Invention
The present invention relates to an improved photo-receiving device for converting light signals to electrical signals, as required in such fields as communications and information processing, and to an improved method of fabricating a photo-device, which also means a light emitting device for converting electrical signals to light signals.
2. Description of the Prior Art
The term photoelectric conversion as used herein refers to the function of converting a light signal to an electrical signal in a photo-receiving device, and also to the function of converting an electrical signal to a light signal in a light emitting device. FIG. 8 illustrates the structure of a conventional photo-receiving device 50 as described in the IEEE Journal of Quantum Electronics (Vol. 28, pp. 2358-2368, 1992) for converting light signals to electrical signals at a relatively high speed. As shown, the photo-receiving device 50 comprises a substrate 11 that constitutes the photoelectric conversion portion (which can also be referred to as a light-absorbing portion, as this is a photo-receiving device), and a pair of opposed electrode portions 51, each in the form of a thin film of metal, formed on the surface of the substrate 11. The exposed surface of the substrate 11 between the electrodes 51 forms an optical window 13 for the entry of the light I.sub.P to be detected.
When light I.sub.P impinges on the optical window 13 while an appropriate voltage is being applied to the pair of electrodes 51, excited carriers (electrons and holes) are generated in the substrate 11. Holes, shown in the drawing as blank circles, are drawn to the electrode 51 with the relatively negative (-) potential, and electrons, shown as solid circles, are drawn to the electrode 51 with the relatively positive (+) potential, setting up a flow of a photoelectric current (photo detection current) which takes place via the electrodes 51, whereby the incidence of the light I.sub.P is detected.
With this type of conventional photo-receiving device 50 of FIG. 8, which is generally referred to as a MSM (metal/semiconductor/metal) device, the smaller the width W of the optical window 13, that is, the smaller the distance between the electrodes 51, the faster the operating speed of the device is, and raising the applied voltage increases device speed and sensitivity. Also, making the width W of the optical window 13 no larger than the wavelength of the light I.sub.P to be detected imparts an evanescent field to the light I.sub.P impinging on the light-absorbing substrate 11 and causes the incident light I.sub.P to be absorbed in the vicinity of the surface of the substrate 11. At the same time, as the field strength set up by the electrodes 51 is higher at the surface of the substrate 11 than in the interior, the excited carriers generated in the vicinity of the surface of the substrate 11 are drawn rapidly to the electrodes 51, enabling higher speed operation to be achieved and the affect of carrier recombination to be reduced.
In the case of the photo-receiving device 50 of FIG. 8, reducing the width W between the electrodes 51 to around 300 nm by means of electron beam lithography, with an existing fine pattern process technology, resulted in a pulse-response full width at half maximum output of 870 fs, which is quite a high speed compared to other photo-receiving devices. However, it is difficult to achieve higher speeds, for the following reasons.
A first problem is that, since the optical window 13 on the surface of the substrate 11 between the electrodes 51 is exposed, applying a higher voltage across the electrodes 51 gives rise to creeping discharge along the exposed surface of the optical window 13 and air-gap discharge, rendering the device unusable. That is, if the width W between the electrodes 51 is reduced beyond a certain limit, even a low voltage causes a dielectric breakdown. On the other hand, even in cases where the width W of the incident light window 13 can be increased, within the limitation that it does not exceed the wavelength of the incident light I.sub.P, there are major constraints on the voltage that can be applied. A second problem is that of the limits of the process technology. Even with existing electron beam lithography, a relatively high precision fine pattern process technology, an electrode gap cannot really be precision-fabricated to a width W of 100 nm or less, and even 300 nm or less is quite difficult.
A conventional method of resolving the first problem is to cover, or bury, the exposed surface portion constituting the optical window 13 between the electrodes 51. This will now be described, with reference to FIG. 9. The following description is not limited to photo-receiving devices, being also applicable to devices, such as light emitting devices, having a light emitting area defined in the form of an optical window. Thus, reference numeral 52 is used to denote the photo-device shown in FIG. 9, the light-absorbing portion 11 of FIG. 8 is the photoelectric conversion portion in the general meaning of the term, and the term metallic film electrodes 51 is encompassed by the term optically nontransparent conductive film (electrodes) 12. To fabricate the photo-device 52, existing lithographic technology is used to remove a prescribed region of an optically nontransparent conductive film 12 formed on photoelectric conversion portion 11, thereby exposing a defined portion that forms an optical window 13.
Sputtering or another such vapor deposition technique is then used to form an optically transparent protective insulation layer 53 over the optical window 13. However, sputtering and other such vacuum vapor deposition apparatuses are costly, so there is no objection to achieving the required result by other means. Also, while the optically nontransparent conductive film 12 in which the optical window 13 region is formed (defined) and the insulation layer 53 to protect the optical window 13 are formed using separate processes, device fabrication can be simplified by effecting both processes in one step. As described, even with the relatively high patterning precision provided by a technology such as electron beam lithography, the minimum width W of the optical window 13 that can be formed in the optically nontransparent conductive film 12 is in the order of 300 nm.
Non-uniformities in the thickness of the surface deposition film gives rise to variation in device characteristics, and high frequency characteristics can be degraded by dielectric deposits. When a very fine optical window is used, forming a high-quality insulative film over the window that has high dielectric resistance is difficult.
An object of the present invention is to provide a high speed photo-receiving device in which constraints relating to the width W of the optical window between electrodes, and to the applied voltage, are reduced.
Another object of the present invention is to provide a photo-receiving device that is more highly functional and multifunctional than conventional photo-receiving devices.
Yet another object of the present invention is to provide a method of fabricating a photo-device wherein an optical window region and a protective layer over the optical window surface can be formed in one step.
A further object of the present invention is to provide a method of fabricating a photo-device that enables the above-described drawbacks to be resolved or alleviated and an optical window to be formed to have a smaller width than that of conventional photo-devices.